The present invention relates generally to silver metallizations and, more particularly, the invention relates to a silver-filled glass composition specifically adapted to bond silicon semiconductive devices to substrates.
Silver metallization compositions had their origins in decorative enamelling, but were adapted early on for use in thick film hybrid circuitry. The attention of early workers was, however, concentrated on designing compositions that would adhere strongly to the ceramic substrate. The so-called "Scotch tape test" became an early standard of adhesion. Knox, U.S. Pat. No. 2,385,580 disclosed high proportions of bismuth oxide in lead borosilicate glasses which was widely used with silver, but was not satisfactory with other noble metals. Hoffman, U.S. Pat. No. 3,440,182 disclosed additions of vanadium and copper oxides as improving adhesion, solderability and conductivity of noble metal metallizing compositions generally. These compositions were used as conductors, rather than as a medium for attachment of devices such as silicon integrated circuits to the substrates.
In the latter category, gold-based inks or preforms have been the most common, taking advantage of the low-temperature gold-silicon eutectic to achieve a good bond. Even though very substantial efforts have been made to reduce the amount of gold used to make such bonds, its expense mitigates against its use wherever possible.
There has been a great deal of effort over the years to eliminate gold from hermetic packages in the electronics industry. One of the more difficult areas to eliminate gold has been in MOS technology, due to the necessity of having backside low resistance contact; as of now, gold is still the material of choice in this application.
Plastic packaging has nearly eliminated the necessity for gold, with the exceptions of gold bonding wire and gold evaporated on the backside of the wafer. The gold on the frame and gold preform have been eliminated by the use of epoxy and polyimides filled with silver flake in such packages.
Silver-filled polyimides have been used for die attachment in hermetic packages. Because of the problem of final cross linking of polyimides, and the generation of CO.sub.2 and H.sub.2 O during sealing, this has not achieved significant volume.
There are no low-temperature phases in the silver-gold system, which is a continuous series of solid solutions, and the silver-silicon system has a eutectic, but a high temperature one (over 800.degree. C.), so systems based on silver must employ a fundamentally different bonding mechanism, indeed one where the silver per se plays little or no part.
Thus, where a gold preform is used to attach a silicon die to a silver metallized surface, the mechanism on one side is the gold-silicon eutectic and on the other it is a solid-liquid diffusion, with the glass playing the major role in terms of bond strength. Being less than a metallurgical bond, the thermal and electrical conductivity are not as good as desired.
Pure glass bonds have also been used in this service, but without a conductive element both conductivities suffer, as would be expected.
Regarding silver polyimide compositions, the quantity of silver that can be incorporated is limited, and special processing is necessary (for high-volume manufacturing, uniformity of processing is an important cost consideration). The biggest drawback of polyimides, or any organic bonding system, is that they can not be used in hermetic packages such as Cerdips, because they are moisture getters, can not be outgassed, and generally can not withstand high temperature used in assembling these packages.
The present invention provides a silver-filled glass that produces strong bonds between the silicon die and the substrate, whether or not the latter is metallized, with controllable thermal and electrical conductivity, and which may be used in hermetic packages.